Remote sensing of gas leaks

ABSTRACT

A gas filter correlation radiometer mounted on an aircraft is flown over a target area. The gas filter correlation radiometer is configured to detect ethane (C 2 H 6 ) gas in the event of a gas leak. The gas filter correlation radiometer uses background radiation to detect ethane.

CROSS-REFERENCE TO OTHER APPLICATIONS

This application claims priority from provisional application U.S.60/455,225 filed Mar. 13, 2003.

BACKGROUND OF THE INVENTION

This invention relates to remote sensing techniques to detect gas leaks.In particular, the invention involves flying an aircraft with a remotesensing instrument over a target area, such as a pipeline, and measuringabsorption of upwelling electromagnetic radiation that has passedthrough natural gas.

Past attempts to remotely detect natural gas leaks have involveddetecting increased concentrations of methane (CH₄). CH₄ comprisesapproximately 95% of the composition of natural gas, which makes it anatural target for detection. One problem that has been experienced isthat CH₄ exists in fairly large quantities in the atmosphere (it is wellmixed in the atmosphere with a concentration of approximately 1.7 ppm).Therefore, detecting a gas leak required detection of a small increaseon a large background. Events such as passing near a source region ofCH₄ (such as a farm), or an increase in the altitude of the airplane (anincrease in the atmospheric path length) might result in the falsesignature of a leak.

To reduce the influence of the background, some past attempts have triedto detect the excess CH₄ of a natural gas leak by detecting theabsorption of CH₄ in the long wavelength infrared region (for example,at 7.8 μm or 2180 cm⁻¹). This provides the advantage that the upwellingradiation is primarily emitted from the earth's surface. This minimisesthe background CH₄, as only the CH₄ located between the airplane and theearth's surface is detected.

However, for underground pipe since the temperature of the surface andthe leaked CH₄ are nearly the same, the radiative contrast between thesurface and the leaked methane is very small, greatly reducing thedetectivity/detectability of the leak. Also, the thermal noiseintroduced within the instrument itself becomes a serious designconstraint. Using a shorter wavelength absorption band of CH₄ couldpotentially help, as the upwelling radiation would be primarily from thesun. This would greatly increase the radiative contrast between thesource and the “leaked” gas, and significantly reduce the thermal noisewithin the instrument. However, the background of CH₄ becomes verylarge, as the solar radiation reaching the instrument would have passedthrough entire atmosphere.

SUMMARY OF THE INVENTION

According to an aspect of the invention, leaks from natural gas aredetected by remote detection of radiation that has passed through aconcentration of ethane gas.

Ethane, C₂H₆, is a minor constituent in natural gas, comprising up to20% of unprocessed natural gas, and approximately 2.5% of processednatural gas. It has the distinct advantage for detection over CH₄ inthat it exists in the atmosphere in very minute quantities (globalannual average concentration of 860 ppt). Therefore, the naturalbackground is very small (2000 times smaller than CH₄).

The two main sources of C₂H₆ in the atmosphere are from leaked naturalgas and biomass burning (both of roughly equivalent magnitudes). Themain sink is by reaction with the hydroxyl (OH) radical. C₂H₆ has astrong absorption/emission band(s) at 3000 cm⁻¹ (3.33 μm). In thisspectral region, the upwelling radiation will consist primarily ofreflected solar radiation. Also, thermal noise within the instrument issignificantly reduced at these shorter wavelengths. This combination ofa tiny background, a strong spectral signature in a “solar” region ofthe spectrum, and reduced thermal noise makes C₂H₆ suitable for thedetection of natural gas pipeline leaks. Preferably, the ethane isdetected by using the ethane absorption/emission band(s) at 3000 cm⁻¹.

According to a further aspect of the invention, the ethane is detectedby tuning the gas filter correlation radiometer to detect ethane usingan ethane absorption peak at a bandwidth of 2850 to 3075 cm⁻¹.

According to a further aspect of the invention, the ethane is detectedby tuning the gas filter correlation radiometer to detect ethane usingan ethane absorption peak at a bandwidth up to 150 cm⁻¹ above or below3000 cm⁻¹.

According to a further aspect of the invention, remote sensing of leakedgas, such as leaked ethane from natural gas pipelines, is carried out byusing a gas filter correlation radiometer (GFCR). In a further aspect ofthe invention, the gas filter correlation radiometer comprises a windowin a housing; optics defining a first optical path and a second opticalpath between the window and a detector section mounted in the housing; abeam splitter mounted in the housing as part of the optics for directingradiation entering the window from an outside source to divide theradiation between the first optical path and the second optical path;the first optical path having a first gas path length and the secondoptical path having a second gas path length, the first gas path lengthbeing different from the second gas path length; and electronics forprocessing signals produced by the detector section section as a resultof radiation being directed by the optics onto the detector section. Thegas path length is provided by gas filters containing differing amountsof the target gas, such as ethane, preferably with one of the opticalpaths having zero gas path length.

According to a further aspect of the invention, the beam splittercomprises a bi-prism.

According to a further aspect of the invention, the detector sectionfurther comprises a first detector on the first optical path and asecond detector on the second optical path, and corresponding pixels onthe first detector and second detector having collocated fields of viewand being sampled synchronously.

According to a further aspect of the invention, the detector sectiondetects radiation using a pushbroom imaging technique.

These and other aspects of the invention are described in the detaileddescription of the invention and claimed in the claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

There will now be described preferred embodiments of the invention, withreference to the drawings, by way of illustration only and not with theintention of limiting the scope of the invention, in which like numeralsdenote like elements and in which:

FIG. 1 is a schematic of the gas filter correlation radiometer;

FIG. 2 is a schematic of an alternative embodiment of the gas filtercorrelation radiometer;

FIG. 3 depicts a helicopter using the gas filter correlation radiometerto detect a leak in a pipeline;

FIG. 4 depicts an overhead view of a helicopter traversing a pipelineand shows successive fields of view, including an exploded view of aportion of a field of view being sampled; and

FIG. 5 shows, upper graph, a spectra of C₂H₆ in a 28.6 mm gas cell with106 Pa of pure C₂H₆, middle graph, a high resolution spectra of C₂H₆ ina 28.6 mm gas cell with 12.1 kPa of pure C₂H₆ and, lower graph, acalculation of the spectra using the Hitran line database.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In this patent document, the word “comprising” is used in itsnon-limiting sense to mean that items following the word in the sentenceare included and that items not specifically mentioned are not excluded.The use of the indefinite article “a” in the claims before an elementmeans that one of the elements is specified, but does not specificallyexclude others of the elements being present, unless the context clearlyrequires that there be one and only one of the elements.

The instrument used in this invention is a type of gas-filtercorrelation radiometer (GFCR). GFCRs have been used in differentconfigurations for over 3 decades in remote sensing instrumentation.

Referring to FIG. 1, there is shown a GFCR 101 incorporated within ahousing 100, with a detector section, such as a pair of photodiodearrays 102A, 102B mounted in the housing. Radiation from source 126passes through a window 103 in the housing 100, is collected bycollector optic 124 and filtered by bandpass filter 116 and thendirected by collimating lens 122 onto beam splitter 106. In an exemplaryembodiment, a 40 cm⁻¹ wide band-pass filter 116 centred at 2988 cm⁻¹ isspecified. The filter width is 1.3% of the central wavenumber. Thepassband of filter 116 is selected to include the ethane absorption peakat 3000 cm⁻¹ and exclude radiation falling outside of the peak. Beamsplitter 106 formed by a partially reflective mirror splits theradiation from the radiation source 126 along paths 110 and 112. On thefirst radiation path 110, the radiation passes through gas correlationcell 114 and is focused by detector lens 104A onto the photodiode 102A.On the second radiation path 112, the radiation is directed by mirror120 through an evacuated gas cell 118 and is focused by lens 104B ontophotodiode 102B. The gas correlation cell 114, also called a gas filteror absorption cell, contains a gas, such as ethane, to be detected.

The gas correlation cell 114 may for example be a 1 cm cell with forexample a concentration of ethane provided by one atmosphere of pureC₂H₆. The second path 112 has a different path length of C₂H₆, such asmay be obtained by providing the cell 118 with for example no C₂H₆, asfor example an evacuated gas cell or a cell containing a gas that isoptically neutral in relation to the ethane spectra of interest. Theoutput of the photodiodes 102A, 102B is provided to suitableelectronics, such as a computer 108, for processing. The GFCR 101 mayuse a beam splitter, for example, in the form of a partially reflectivemirror as shown in FIG. 1, or in the form of a bi-prism, as shown inFIG. 2, or may selectively direct the incoming radiation throughseparate paths, in a time division manner, using for example a chopper.The use of a beam splitter versus a chopper is a trade-off betweensimultaneity of the two received signals and loss of signal intensity. Abeam splitter, such as a partially reflective mirror or a bi-prism, ispreferred for gas leak detection because it provides simultaneousmeasurement of both detector signals. This can be important because thesignals are fast varying due to the forward motion of the helicopter andthe variation in the reflective surface.

A different optical configuration is shown in an alternative embodimentin FIG. 2. Radiation from source 226 passes through a window 203 inhousing 200, is collected on collector optic 224 and focused to a fieldstop 216. The field stop 216 is used to limit the field of view. Theradiation from source 226 is then directed by collimating lens 222 ontoprisms 206 and 207 which form the front of a compound gas cell 215formed by gas cell walls 228, gas cell separator 230, and a planeparallel gas cell window 232. The prisms 206 and 207 split the radiationfrom the radiation source 226 along paths 210 and 212 by causing theradiation to diverge while passing through gas cells 214 and 218. On thefirst radiation path 210, the radiation is directed by prism 206 throughgas correlation cell 214 and is focused by detector lens 204 onto thephotodiode 202A. On the second radiation path 212, the radiation isdirected by the prism 207 through an evacuated gas cell 218 and isfocused by detector lens 204 onto photodiode 202B.

The compound gas cell 215 with prisms 206 and 207 may also be locatedbetween the field stop 216 and the collimating lens 222, or between thedetector lens 204 and the photodiodes 202A and 202B. Likewise, theprisms 206 and 207 may be located at either the front of the compoundgas cell 215 or at the back of the compound gas cell 215.

The gas correlation cell 214, also called a gas filter or absorptioncell, contains a gas, such as ethane, to be detected. The gascorrelation cell 214 may for example be a 1 cm cell with for example aconcentration of ethane provided by one atmosphere of pure C2H6. Thesecond path 212 has a different path length of C2H6, such as may beobtained by providing the cell 218 with for example no C2H6, as forexample an evacuated gas cell or a cell containing a gas that isoptically neutral in relation to the ethane spectra of interest. Theoutput of the photodiodes 202A, 202B is provided to suitableelectronics, such as computer 208, for processing.

The detector signal on the path 112 is:

S₁ = G∫_(λ₁)^(λ₂)I_(λ)τ_(filter) λ

where I_(λ) is the intensity of the radiation from the radiation source126, τ_(filter) is the transmissivity of the filter 116, λ₁ is the lowpass of the filter 116, λ₂ is the high pass of the filter 116 and G isthe gain of the photodiode 102B.

The detector signal on the path 110 is:

S₂ = G∫_(λ₁)^(λ₂)I_(λ)τ_(filter)τ_(corr.cell) λ

where τ_(corr cell) is the transmissivity of the correlation cell 114.

If

$S_{avg} = \frac{S_{1} + S_{2}}{2}$

and S_(diff)=S₁−S₂ then the calculation made by the computer is:

${S_{inst} = \frac{S_{diff}}{S_{avg}}},$

which yields a signal that is dependent on the presence of the targetgas in the radiation path from the source 126 to the photodetector 102B.The calculation of the difference in the received signals for bothoptical paths is made for each pixel of the photodetectors 102A, 102B toyield an image of the field of view that may be displayed on a monitor.

FIG. 3 shows the manner of use of the GFCR 101 shown in FIG. 1. Ahelicopter 350 traverses a pipeline 354 with a GFCR 101 having a fieldof view 352 oriented towards the pipeline 354. The GFCR 101 is tuned todetect ethane by appropriate selection of the bandpass of the filter116, and the gas filter 114 contains a sample of ethane. If a leak 356exists in the pipeline 354, the presence of ethane in the resultingplume 358 that may be moved by the presence of wind 360 will be detectedusing the GFCR 101. The presence of a leak is indicated by for exampledisplaying the received signal using a monitor that is provided as partof the computer 108. Pixels on the monitor display corresponding todetected ethane may be coloured to enhance the image. Other methods ofindication of the presence of a leak may be used such as detecting aconcentration of ethane in the path between helicopter 350 and theground that exceeds a predetermined threshold, and providing an alarm.The threshold is determined readily by calibration of the radiometer andexperimentation. FIG. 5 shows that the absorption spectra of ethane at3000⁻¹ cm is larger than the calculated spectrum from the Hitrandatabase, with the result that ethane is unexpectedly a suitablecandidate for the detection of pipeline gas leaks. Due to the remotesensing capabilities of the device, the GFCR does not have to flythrough the plume in order to detect leaks. The GFCR measures theintegrated column concentration of natural gas between the helicopterand the ground, regardless of where in this column the natural gasoccurs.

In one embodiment as shown in FIG. 4, the field of view 352 covers anarea of 128 m², representing a swath 64 m long by 2 m wide. The long butnarrow swath of the field of view 352 leads to an overall view of thepipeline 354 or target area through the use of a technique known aspushbroom imaging. As the helicopter 350 advances along the helicopterpath 464 over the pipeline 354 or other target area, successive swathsbelow the helicopter 350 and perpendicular to the helicopter path 464are detected by the GFCR 101. At a first time interval, the detectors102A and 102B would sample signals from the field of view 352A, followedmoments later by 352B, followed again by 352C and so on.

In FIG. 4, the field of view 352F represents the current swath of thetarget area being detected by the detectors 102A and 102B. Detectors102A and 102B have corresponding pixels having collocated fields of view352F where each 2 m×2 m cell of the field of view 352F is sampledsynchronously by detectors 102A and 102B. Therefore, the cell marked P1would be detected by a first pixel representing a portion of the fieldof view collocated and synchronized on detectors 102A and 102B. The cellmarked P2 would be detected by a second pixel collocated andsynchronized on detectors 102A and 102B. The same can be said for thecells marked P3 and P4 and so on. All cells P1 to P32 along a line wouldbe detected simultaneously.

In an exemplary embodiment, the GFCR 101 operates using ambientbackground radiation that passes through the plume 358 of natural gas.The upwelling radiation field is comprised of reflected solar radiation,radiation emitted from the surface, plus upwelling emission from theatmosphere. For operation during cloudy periods or at night, a source ofillumination 362 may be used. For example, a powerful 1600 W TungstenHalogen bulb may be mounted on the helicopter 350, with an IRtransmitting window (not shown) and a focusing mirror (not shown). Thismirror focuses the emission from the illumination source 362 to a 5 mspot on the ground. Assuming a lambertian reflective surface and areflectivity of 5%, the reflected intensity at the surface would be0.048 W m⁻². This is roughly equivalent to (or slight greater than) thereflected intensity of sunlight. The illumination source 362 should bemounted to reduce vibrations that could increase the signal to noiseratio of the detected signal. In an alternative embodiment, the GFCR 101may be mounted on a different type of vehicle, such as a truck, anddriving the vehicle along a pipeline or other possible source of a gasleak. The GFCR 101 may also be tuned to detect other gases by selectionof the bandpass of the filter 116.

The detected instrument signal is a function of the height of thenatural gas column. For an atmospheric background concentration of 1 ppbof C₂H₆, the equivalent total atmospheric column thickness isapproximately 8.5 μm. The equivalent CH₄ column thickness would beapproximately 1700 times thicker.

A linear regression of the signal sensitivity between 0 and 4 mm ofnatural gas shows that the change in signal per min of natural gas is−1.69×10⁻³ mm⁻¹. The measurement is actually detecting C₂H₆ which isassumed to be 2.5% of natural gas. Therefore, the detected columns ofpure C₂H₆ are 40 times shorter than that of methane. Maximum sensitivityto C₂H₆ occurs at the lowest concentrations. This is the most desirablefor detecting the smallest leaks.

Uncertainties may be introduced into the measurement by spectralinterferences by other gases in the atmosphere (principally H₂0 andCH₄), variations in the surface emissivity, temperature variations inthe atmospheric temperature, and variations in the altitude of theairplane. These uncertainties tend to reduce the sensitivity of themeasurement to concentrations of natural gas, and variations may resultin false signatures of leaks. The combined uncertainty is about +/−19μm. This level of accuracy places a minimum limitation on themeasurement's accuracy. Given a measurement resolution of −1.69×10⁻³ permm natural gas, to measure a column height of ±19 μm a measurementprecision of ±3.2×10⁻⁵ (i.e. a signal-to-noise ratio of 31,000) isrequired. Such a measurement precision may be obtained from the GFCR101, and may be adjusted by for example varying the length of theabsorption cell 114.

The sensitivity of the instrument is ultimately a function of the amountof energy that is collected and focussed onto the detector element. Thisin turn is a function of the field-of-view (FOV) of the instrument(which determines the surface resolution), the size of the collectoroptic 124, the size of the detector pixel in the photodiodes 102A, 102B,the transmission of the instrument, and the observation period(frequency) of the instrument. The FOV and the collector optic sizedirectly affect the energy collected, as the larger the optic and FOV,the more photons collected. However, they also directly affect thedetector pixel size, due to the principle of etendue (A^(Ω))conservation in an optical chain. The transmission of the instrumentdirectly affects the energy collected as any losses in the systemdirectly reduces the number of photons incident on the detector. Andfinally, the pixel size and observation period directly affect thenoise-equivalent power (NEP) of the detector. In an exemplaryembodiment, the aircraft may operate at a height of 30 m, with surfaceresolution 1.5 m, FOV solid angle 2.0×10⁻⁵ m² sr, FOV 2.86°, collectoroptic diameter 12.2 cm, A^(Ω) product 2.29×10⁻⁵ m² sr, transmission 75%,temperature 293K, observation time 10 ms (100 hz), detector elementdiameter 2 mm, detector FOV 170° and detector D*10¹¹ cm Hz^(0.5).

The upwelling radiance reaching the aircraft is calculated to be 0.04 Wm⁻² sr⁻¹. This includes the energy lost due to absorption by atmosphericH₂0 and CH₄, and which is reduced to 0.03 W m-2 sr⁻¹. Assuming theinstrument has a 12.2 cm diameter optic to collect upwelling radiationwith a field-of-view of 2.86° and an instrument transmission of 75%, thecollected energy by the instrument will be 5.2×10⁻⁷ W. The noiseequivalent power (NEP) for a 2 mm diameter liquid nitrogen cooled InSbdetector would be 2×10⁻¹¹ W, providing a radiative S/N ratio ofapproximately 25,800. Given this level of precision and the calculatedsensitivity to natural gas of −1.69×10⁻³ mm⁻¹, the measurement is ableto detect below a 23 μm column of natural gas.

A person skilled in the art could make immaterial modifications to theinvention described in this patent document without departing from theinvention.

1-17. (canceled)
 18. A gas filter correlation radiometer, comprising: awindow in a housing; optics defining a first optical path and a secondoptical path between the window and a detector section mounted in thehousing; a bi-prism beam splitter comprising a pair of side-by-sideprisms mounted transversely in the housing in relation to the firstoptical path and the second optical path as part of the optics fordirecting radiation entering the window from an outside source along twodivergent paths offset from each other by refraction through thebi-prism beam splitter to divide the radiation between the first opticalpath and the second optical path; the first optical path having a firstgas path length and the second optical path having a second gas pathlength, the first gas path length being different from the second gaspath length; and electronics for processing signals produced by thedetector section as a result of radiation being directed by the opticsonto the detector section.
 19. (canceled)
 20. The gas filter correlationradiometer of claim 18 in which the gas filter correlation radiometer istuned to detect ethane using the ethane absorption peak at 3000 cm⁻¹.21. A gas filter correlation radiometer, comprising: a window in ahousing; optics defining a first optical path and a second optical pathbetween the window and a detector section mounted in the housing; a beamsplitter mounted in the housing as part of the optics for directingradiation entering the window from an outside source to divide theradiation between the first optical path and the second optical path;the first optical path having a first gas path length and the secondoptical path having a second gas path length, the first gas path lengthbeing different from the second gas path length; and electronics forprocessing signals produced by the detector section as a result ofradiation being directed by the optics onto the detector section, thegas filter correlation radiometer being tuned to detect ethane using anethane absorption peak at a bandwidth of at least 2850 to 3075 cm-1. 22.The gas filter correlation radiometer of claim 21 in which the gasfilter correlation radiometer is tuned to detect ethane using an ethaneabsorption peak at a bandwidth up to 150 cm⁻¹ above or below 3000 cm⁻¹.23. The gas filter correlation radiometer of claim 21 in which the gasfilter correlation radiometer is tuned to detect ethane using the ethaneabsorption peak at 2850 to 3075 cm⁻¹ by incorporating a filter in theoptics that selects radiation in a passband that includes the ethaneabsorption peak at 2850 to 3075 cm⁻¹.
 24. The gas filter correlationradiometer of claim 18 in which the first optical path incorporates agas filter containing ethane.
 25. The gas filter correlation radiometerof claim 24 in which the second gas path length is lower than the firstgas path length. 26-27. (canceled)
 28. The gas filter correlationradiometer of claim 18 in which each prism of the side-by-side prismshas a thinner side and a thicker side, the pair of side-by-side prismsbeing joined along the respective thinner sides.
 29. The gas filtercorrelation radiometer of claim 18 further comprising a source ofillumination mounted with the gas filter correlation radiometer on boardan aircraft.
 30. The gas filter correlation radiometer of claim 21further comprising a source of illumination mounted with the gas filtercorrelation radiometer on board an aircraft.